Are you saying that there is a capacitor between input? output? of the
79L05 and the ground pins of the chips? If so that would seem pretty
normal as it is usually recommended that you have capacitors to ground
on both the input and output side of a 3 terminal regulator. Are the
ground pins on the three chips you mentioned isolated from the ground of
the rest of the board?
On 2018-11-02 10:09 PM, Adrian Graham via cctalk wrote:
Hi folks,
This week I managed to get my paws on a machine that I only ever saw in ‘coming
up!’ type magazine articles in the mid-80s. It’s made by a UK manufacturer of
Viewdata set top boxes and home micro modems called Tandata who were a split
from Tangerine, the company that gave us the Microtan 65 and eventually the
Oric 1 and Oric Atmos.
Documentation on the Tandata PA is zero, if you search for it you get my Binary
Dinosaurs page and nothing else so tonight I set about trying to work out the
power inputs from its 4 pin socket. Going clockwise pin 1 is definitely GND/0V
and pin 2 is not connected. Pin 3 goes to the input of a 79L05 -5V regulator
which via a capacitor seems to be used as the GND pins for 3 CMOS 74 series
chips. Pin 4 goes to a 7805 5V regulator.
I’ve never seen a -5V reg be used in a GND circuit so before I continue
searching am I barking up the wrong tree? The trace literally goes from socket
to 79L05 pin 2, output goes to a capacitor then to the GND pins on a CD74HC74E,
CD74HC86E and CD74HC4066E. There’s a VARTA battery nearby too.
Board pic is here: http://binarydinosaurs.co.uk/tandatapa-13.jpg
<http://binarydinosaurs.co.uk/tandatapa-13.jpg>
Any insight much appreciated!
